Determination of the acid value and the acidity using standard method
Scope
This standard describes method for the determination of the free fatty acid content of animal
and vegetable oils and fats. This content can be expressed as an acid value, or as acidity.
Field of application
This method is applicable to animal and vegetable oils and fats. It is not applicable to waxes.
Definition
The acid value is the number of mg of potassium hydroxide required to neutralize the free
fatty acids in 1 g of the fat.
The acidity is an expression of the content (in %, m/m; percentage) of free fatty acids as
content of dominant or chosen fatty acid. According to the nature of the fat it can be
expressed as in Table 1.
Table 1
Nature of fat
Coconut, palm kernel and similar oils
Palm oil
All other oils
Expressed as
Lauric acid
Palmitic acid
Oleic acid
Molecular weight
200
256
282
When the result is reported as acidity, but not further defined, then conventionally the acidity
is always expressed as oleic acid.
If the sample contains mineral acids, by convention they are calculated as fatty acids.
Principle
Solution of a known quantity of the fat to be analysed in a mixture of ethanol and diethyl
ether, followed by titration of the free fatty acids present with an ethanolic solution of
potassium hydroxide.
Apparatus
- burette
- titration vessels
Reagents
- solvent mixture 1/1 (V/V) of 95 per cent (V/V) ethanol and diethyl ether
- potassium hydroxide, about 0.1 mol/l solution in ethanol
(the exact concentration should be known)
- phenolphthalein, 10 g/l solution in 95 per cent (V/V) ethanol
Procedure
Determine size of sample according to the Table 2.
Table 2
Expected acid value
<1
1-4
4-15
15-75
>75
Mass of test portion, in g
20
10
2.5
0.5
0.1
Weighing accuracy, in g
0.05
0.02
0.01
0.001
0.0002
Weigh the test portion into a titration vessel.
Dissolve it in 50 ml of the solvent mixture of ethanol and diethyl ether.
Add 0.2 ml of phenolphthalein solution and titrate, with shaking, with the solution of
potassium hydroxide in ethanol to the pink colour persisting for at least 10 seconds.
Carry out simultaneously a blank test.